Simplify error.c without error addition

[gilles-peskine-arm]

Since #9926, error codes can no longer be the sum of a low-level code and a high-level code. They're all MBEDTLS_ERR_xxx constants (or PSA_ERROR_xxx, but that's a separate issue).

The goal of this issue is to simplify error.c (technically error.fmt) and generate_errors.pl accordingly. The current code still copes with values that are the sum of a low-level code and a high-level code, but this is no longer meaningful. We just have one set of numerical constants to translate to string constants.

We may or may not want to handle PSA_ERROR_xxx values at the same time.